Pulled up the xylem, and wilting — questions

1. identify

Supplement. A leafy shoot with no roots, in a potometer, still takes up water. What moves the water up its xylem?

Circle one answer.

• Living cells in the xylem walls squeeze the water up the stem
• The air bubble in the potometer pushes the water up the stem
• Pressure from the roots pushes the column of water up the stem
• Water evaporating from the leaves pulls the column of water up

Nudge (only if you are stuck): The shoot has no roots. Where is water being lost?

2. sequence

Supplement. Put these stages of the transpiration pull in order, starting in the leaf.

Write the number of each step in order.

1. The mesophyll cells take in water from the xylem
2. More water enters the roots from the soil
3. Water evaporates from the mesophyll cells and the vapour diffuses out of the leaf
4. The whole column of water in the xylem is drawn up

• First: ______
• Second: ______
• Third: ______
• Last: ______

Nudge (only if you are stuck): Start with the water that leaves the leaf.

3. explain

Supplement. Why does the water in a xylem vessel move up as one whole column, rather than breaking apart?

Circle one answer.

• Water molecules attract one another, so the column holds together
• Valves in the xylem vessels stop the water from falling back down
• Air from the potometer bubble pushes the whole column up from below
• The xylem walls are sticky, so they pull the whole column up

Nudge (only if you are stuck): What do water molecules do to one another?

4. state

Supplement. When does a plant wilt?

Circle one answer.

• When it uses up its food faster than its leaves can make it
• When its cells take in more water than they can hold
• When it loses water faster than its roots can take water up
• When its leaves get more light than they can use

Nudge (only if you are stuck): Compare water out with water in.

5. state

Supplement. Complete the sentences about how water moves up the xylem.

Water … from the mesophyll cells in the leaves.

This causes a … pull

that draws up a … of water molecules in the xylem.

The molecules are held together by forces of … between them.

Nudge (only if you are stuck): One gap is a change of state; one names the pull; one is what the water forms; one is what holds it together.

6. explain

Supplement. When you set up a potometer, you cut the shoot and fit it into the potometer under water. Explain why.

Nudge (only if you are stuck): What would get into the cut ends of the xylem in air?

7. sequence

Supplement. Put the stages of wilting in order.

Write the number of each step in order.

1. The cells lose water and their vacuoles shrink
2. The cells become flaccid
3. The leaves and stems droop
4. Water is lost by transpiration faster than the roots take it up

• First: ______
• Second: ______
• Third: ______
• Last: ______

Nudge (only if you are stuck): Start with the water balance of the whole plant.

8. identify

Supplement. Choose all the conditions that make a plant more likely to wilt.

Tick every correct statement.

• Dry air
• Cool, still, humid air
• Hot air
• A strong wind
• Wet soil
• Dry soil

Nudge (only if you are stuck): Which conditions make transpiration faster, or uptake slower?

9. explain

Supplement. A sunflower in moist soil wilts on a hot, sunny, windy afternoon, but its leaves are firm again by the next morning, without watering. (a) Explain why it wilted in the afternoon. (b) Suggest why it recovered overnight.

(a) Why it wilted

(b) Why it recovered

Nudge (only if you are stuck): Compare transpiration with uptake in the afternoon, then at night.

10. predict / explain

Supplement. All the leaves are cut off a young plant growing in moist soil. Predict what happens to the movement of water up its stem, and explain why.

Circle one answer.

• It flows back down, because the leaves no longer hold it up
• It slows almost to a stop, because nothing pulls the water up
• It stays the same, because the roots keep pushing the water up
• It speeds up, because the water no longer goes into the leaves

Nudge (only if you are stuck): Where does the pull come from?

11. explain

Supplement. Florists say that cut flower stems should be cut again, under water, before the flowers go into a vase. Flowers whose stems were left in the air for a long time often droop, even in a full vase. (a) Explain why the flowers droop. (b) Explain how cutting the stems again under water helps.

(a) Why the flowers droop

(b) How cutting under water helps

Nudge (only if you are stuck): What gets into a cut stem left in the air?

12. explain

Supplement. When seedlings are moved from a seed tray into the garden, many of their root hairs are torn off, and they often wilt. Gardeners move them in the evening or on a cloudy day. Explain why the seedlings wilt, and why moving them in the evening helps.

Nudge (only if you are stuck): What do root hairs do? And what happens to transpiration in the evening?

13. predict

Supplement. Every leaf of a shoot in a potometer is coated on both surfaces with petroleum jelly, which water vapour cannot pass through. What happens to the bubble, and why?

Circle one answer.

• It moves backwards, because the leaves push the water out
• It keeps the same rate, because the stem still takes up water
• It moves faster, because water builds up inside the leaves
• It almost stops, because no water vapour can leave the leaves

Nudge (only if you are stuck): Can water still evaporate and diffuse out of the leaves?